The use of zinc electrodeposits on iron or steel substrates to provide improved corrosion protection has long been practiced. Although such zinc electrodeposits greatly enhance the corrosion resistance of the iron or steel substrate, the zinc itself forms white "rust" or corrosion which, ultimately, can result in the corrosion of the iron or steel substrate itself with the consequent formation of red rust. To minimize this and improve the corrosion resistance of the zinc electroplated substrates, it has commonly been the practice to treat such surfaces with an acidic, hexavalent chromium containing solution to form a visible or colored chromate passivating film on the surface. Typical compositions and process for forming such chromate passivating films are disclosed in U.S. Pat. Nos. 2,021,592; 2,106,904; 2,288,007; 2,376,158; 2,939,664; 2,610,133; 2,760,891; 3,090,710; 3,404,046; and 3,895,969.
Recently, considerable work has been done to improve the corrosion resistance of zinc electroplated substrates by the substitution of zinc alloy electrodeposits for the substantially pure zinc electrodeposits which have heretofore been used. Although various different metals have been used with the zinc in such alloy electrodeposits, particularly good results, in terms of the improvements in corrosion resistance and the brightness or gloss of the surface, have been obtained with zinc-nickel alloy electrodeposits. Depending upon the nickel content of such zinc-nickel alloy electrodeposits, the time for the formation of red rust in salt spray testing can be as much as five to ten times greater than the time for such red rust formation with zinc electrodeposits. In spite of this, the formation of white rust or corrosion on such zinc-nickel alloy deposits is still a problem which must be minimized by the application of a chromate or other passivating film.
It has been found, however, that the formation of a colored or visible chromate film having high corrosion resistance on such zinc-nickel alloy electrodeposits is much more difficult than on zinc electrodeposits. In the various patents disclosed hereinabove, there is no specific disclosure of the treatment of zinc-nickel alloy electrodeposits, although the treatment of zinc alloy deposits, broadly, is mentioned. Generally, the chromating compositions of these patents have not been effective in producing a satisfactory corrosion resistant chromate film on zinc-nickel alloy electrodeposits. For example, in U.S. Pat. No. 2,106,904, there is disclosed a chromic acid/sulfuric acid solution which contains from about 13 to 104 g/l hexavalent chromium and from about 1.8 to 144 g/l SO.sub.4, with a weight ratio of hexavalent chromium: SO.sub.4 of 0.09-59:1. This solution is specified as having a pH which is not in excess of 1.0, and when used for the treatment of zinc-nickel alloy electrodeposits, the resulting chromate film has relatively poor corrosion resistance. This difficulty in forming colored chromate film with high corrosion resistance on the zinc-nickel alloy electrodeposits is one reason why such deposits have not been more widely adopted as a replacement for zinc electrodeposits for the production of decorative and corrosion resistant coatings on iron and steel substrates.
It is, therefore, an object of the present invention to provide an improved chromating composition which will produce excellent colored, corrosion resistant coatings on zinc-nickel alloy electrodeposits.
A further object of the present invention is to provide an improved process for the formation of colored, corrosion resistant chromate films on zinc-nickel alloy electrodeposits.
These and other objects will become apparent to those skilled in the art from the description of the invention which follows.